1. Field of the Invention
The present invention relates to a technique of an electric storage device, and more particularly, it is well adaptable to a positive electrode of a lithium ion electric storage device.
2. Description of the Related Art
The technique described below has been studied for completing the present invention. The summary is as follows.
In recent years, the environmental issue, particularly the vehicle-exhaust gas emission, is widely talked about. In this situation, efforts are made of developing environment-friendly electric vehicles and the like. In the electric vehicle development, the strong development effort is focused on the electric storage device to be used as a power source. Many types of electric storage devices have been proposed for replacement of the conventional lead battery. Among these electric storage devices, an electric double layer capacitor, which is excellent in rapid regeneration capability of energy, is now attracting attention as a technique capable of being reducing an amount of CO2 exhaust emission with the efficient use of energy. However, the electric double layer capacitor has a problem in the energy density. Therefore, an electric double layer capacitor having more than 10 Wh/kg has not yet been put to practical use.
A lithium ion capacitor described below has been proposed in order to overcome the drawback of the electric double layer capacitor. In the lithium ion capacitor described above, an activated carbon is used for the positive electrode, a carbon material that can storage and desorb lithium ions is used for the negative electrode, aprotic organic electrolytic solution containing lithium salt is used for the electrolytic solution, and lithium ions are pre-doped into the positive electrode and/or negative electrode so as to increase voltage. The lithium ion capacitor described above has an energy density exceeding 10 Wh/kg, and has a high output density and cycle characteristic equal to or more than those of the electric double layer capacitor. A further research and development have been made.
For example, there has been proposed, in the Japanese Unexamined Patent Publication No. 2006-286923, that a technique in which an activated carbon grain described below is used as a positive electrode active material of a lithium ion capacitor. In the activated carbon grain, the pore volume of a pore radius within the range of 0.1 to 4 nm is 80% or more of the total pore volume, and the total pore volume falls within the range of 0.4 to 1.5 cc/g. The Patent Document 1 describes that, when this structure is employed, the energy density and the output density of the lithium ion capacitor can further be enhanced.
As described above, a further study has been made in which the property of the electrode is enhanced by limiting the pore volume of the active material, which constitutes the electrode, to a specific range. However, the pore volume described in the Patent Document 1 is specified within a very wide range. Specifically, if the pore volume of a pore radius within the range of 0.1 to 4 nm is 80% or more of the total pore volume, activated carbons outside the specified range are hardly present in the world.
When an activated carbon is used as the electrode material of the electric double layer capacitor, it is preferable that an activated carbon having a specific surface area of 1500 m2/g or more for exhibiting high capacitance is used. However, when the specific surface area is not less than 1500 m2/g, the pore volume of the general activated carbon becomes 0.4 cc/g or more in general. Specifically, it falls within the range of 0.4-1.5 cc/g of the total pore volume, which is specified in the Patent Document 1.
As described above, the pore volumes of the activated carbons having the specified range in the Patent Document 1 are mostly overlapped with the pore volumes of known activated carbons. Therefore, it is considered that the intended effect cannot be obtained. It is desired that a positive electrode active material of a lithium ion capacitor excellent in input/output characteristic can reversibly and promptly adsorb and desorb anions and lithium ions. It is conceived that the activated carbon having a wide specified range active material described above cannot afford the expected high output density. Further, within the described range of the pore diameter, it is considered that satisfactory characteristic in the load characteristic at low temperature, particularly at the input side, cannot be obtained.
An object of the present invention is to provide a technique of enhancing a characteristic of a lithium ion electric storage device upon charging or discharging with high load, and increasing a working temperature range thereof.
The foregoing and other objects and novel features of the present invention will be apparent from the description of the specification of the present application and the attached drawings.